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Glossary of Common Terms & Gem Terminology

Throughout the AGS Laboratories site, you might find unfamiliar terms. The information below provides a high-level overview of some of the common terms we use to describe how and what we evaluate and grade through AGS Laboratories.

The ASET is a proprietary viewing device created by the American Gem Society. The ASET color-codes the angular ranges from which a diamond draws or gathers light.

Brightness is the amount of white light a diamond gives off.

Brilliance factors in both a diamond’s brightness and contrast. When a diamond gives off a large amount of light through the table, or the flat facet at the top of the diamond, it is considered brilliant. When some facets are darker than others in intense lighting (think sunlight), the diamond will appear brighter.

Contrast is the effect created by the light and dark patterns seen when observing a faceted diamond. Contrast can be positive or negative in its optical effect based on the amount and distribution. Usually it is caused by, but not limited to, the observer’s head.

The Crown is the uppermost part of a diamond above the Girdle.

The Culet is the point on the bottom of a diamond’s pavilion. A diamond’s Culet may be pointed, or it may be blunted with a small facet.

Dispersion is the separation of white light into colors by the refraction in a diamond. Also known as fire.

Durability is a diamond’s resistance to chipping or breaking.

Fire is the amount of rainbow colors coming from a diamond caused by dispersion.

The Girdle is the widest part (outer edge) of the diamond.

Leakage is defined as the areas in a diamond that do not return light.

Light Performance is an analysis of brightness, dispersion, leakage, and contrast in a diamond.

The Pavilion is the lower part of the diamond below the Girdle. 

Polish is the quality and condition of the diamond’s surface and edges.

Proportions are the size and angular relationships among the main parts of the diamond. AGS Laboratories diamond graders consider the following factors when evaluating Proportions: Table Size, Crown Angle, Girdle Thickness, Pavilion Angle, and Culet Size.

Shape is the outline of the diamond, like marquise, pear, or oval. Don’t confuse it with Cut — many people do.

Symmetry is the exactness of the cut design’s execution, including the balance and evenness of the diamond’s shape and placement of the facets.

The Table is the flat facet in the top part of a diamond.

To understand Tilt, you need to know these three terms: Girdle, Table, Pavilion. With these terms in mind, Tilt is the point at which the girdle reflects in the table of a diamond. It is defined by the minimum Pavilion angle allowed for each table size.

The Weight Ratio is the expected weight of a diamond based on the millimeter footprint of a diamond versus its weight.

Have questions or comments? Contact AGS Laboratories.